Test updates for strict initialization #62958

DanielRosenwasser · 2026-01-07T00:24:12Z

As part of #62333, this pull request updates a handful of tests so that they will not encounter the error

Variable 'x' is used before being assigned.

when we switch --strict on by default.

The typical changes you should expect are

a new declare modifier on the variable
a ! definite-assignment assertion
an initializer for the variable
occasionally an explicit strictness flag turned off

Some tests are left as-is though.

Copilot

Pull request overview

This pull request updates TypeScript test baseline files to prepare for enabling --strict mode by default. The changes add declare modifiers to variable declarations that lack initializers, preventing "Variable 'x' is used before being assigned" errors when strict initialization checking is enabled.

Key Changes:

Added declare modifier to uninitialized variable declarations in test baselines
Updated line number references in symbol tables and error messages to reflect the position changes
Removed generated JavaScript variable declarations for declared variables (as they don't emit code)

Reviewed changes

Copilot reviewed 296 out of 2293 changed files in this pull request and generated 6 comments.

File	Description
Multiple `.types` files	Added `declare` modifier to variable declarations in type baseline files
Multiple `.symbols` files	Updated symbol declaration references to reflect new line positions after adding `declare`
Multiple `.js` files	Removed variable declarations that are now declared (don't emit to JavaScript)
Multiple `.errors.txt` files	Updated line numbers and added new errors for `declare` modifier usage in function scopes

Copilot · 2026-01-07T00:25:45Z

tests/baselines/reference/assignmentCompatWithStringIndexer3.errors.txt

+assignmentCompatWithStringIndexer3.ts(7,16): error TS2304: Cannot find name 'A'.
+assignmentCompatWithStringIndexer3.ts(18,9): error TS1184: Modifiers cannot appear here.
+assignmentCompatWithStringIndexer3.ts(19,9): error TS1184: Modifiers cannot appear here.


The baseline file shows new TS1184 errors about modifiers not being allowed inside function scopes. The declare modifier cannot be used inside function bodies - it's only valid at the top level or in ambient contexts. These test cases with declare var inside the foo function are invalid and will fail when strict mode is enabled.

Copilot · 2026-01-07T00:25:45Z

tests/baselines/reference/assignmentCompatWithStringIndexer2.errors.txt

+assignmentCompatWithStringIndexer2.ts(44,9): error TS1184: Modifiers cannot appear here.
+assignmentCompatWithStringIndexer2.ts(45,9): error TS1184: Modifiers cannot appear here.


Similar to Comment 1, the baseline shows TS1184 errors for declare modifiers inside the function scope. These need to use definite assignment assertions (!) or initializers instead of declare when inside function bodies.

Copilot · 2026-01-07T00:25:46Z

tests/baselines/reference/assignmentCompatWithNumericIndexer3.errors.txt

+assignmentCompatWithNumericIndexer3.ts(31,9): error TS1184: Modifiers cannot appear here.
+assignmentCompatWithNumericIndexer3.ts(32,9): error TS1184: Modifiers cannot appear here.


The baseline shows TS1184 errors for declare modifiers used inside function scopes. The declare modifier is not valid inside function bodies and should be replaced with definite assignment assertions or initializers.

Copilot · 2026-01-07T00:25:46Z

tests/baselines/reference/assignmentCompatWithNumericIndexer2.errors.txt

+assignmentCompatWithNumericIndexer2.ts(30,9): error TS1184: Modifiers cannot appear here.
+assignmentCompatWithNumericIndexer2.ts(31,9): error TS1184: Modifiers cannot appear here.


The baseline includes TS1184 errors showing that declare modifiers cannot appear inside function bodies. These variables should use definite assignment assertions or be given initializers instead.

tests/baselines/reference/assignmentCompatWithNumericIndexer.errors.txt

...selines/reference/assignmentCompatWithGenericCallSignaturesWithOptionalParameters.errors.txt

…nitialization

DanielRosenwasser · 2026-01-08T02:12:53Z

I've reviewed the new error diffs in *.errors.txt matching [+-]\!\!|! and I don't see anything that jumps out.

DanielRosenwasser · 2026-01-08T02:13:17Z

@copilot please review

Copilot · 2026-01-08T02:13:32Z

@DanielRosenwasser I've opened a new pull request, #62964, to work on those changes. Once the pull request is ready, I'll request review from you.

DanielRosenwasser · 2026-01-08T02:13:51Z

no no no no

Copilot

Pull request overview

Copilot reviewed 299 out of 2295 changed files in this pull request and generated 2 comments.

tests/baselines/reference/assignmentCompatWithStringIndexer.types

tests/baselines/reference/assignmentCompatWithStringIndexer.errors.txt

Copilot

Pull request overview

Copilot reviewed 299 out of 2295 changed files in this pull request and generated no new comments.

jakebailey · 2026-01-09T20:43:52Z

I am sort of confused why these changes are needed to switch strict; can you give an example of changes to look at or why they differ?

DanielRosenwasser · 2026-01-09T22:39:15Z

@jakebailey our test suite has 2 issues that don't work with --strictNullChecks or --strictPropertyInitialization

Variables are assumed to be initialized just so we have a value of a given type
Class properties are assumed to be initialized for the same reason, or to avoid writing a constructor function

So the usual 4 solutions are the following:

When valid, adding a declare modifier is almost always the preferred solution from what I've seen in most tests.
For local variables and class properties, add a definitely-assigned assertion (!).
If the expression is trivial enough and it does not impact the control flow analysis of types, add an initializer.
Change the strictness settings.

jakebailey

what more can one say about a 1000+ file PR than "LGTM"

DanielRosenwasser added 2 commits January 7, 2026 00:14

Fix a few tests that have used-before-assigned errors.

f6f4618

Accepted baselines.

69c7272

Copilot AI review requested due to automatic review settings January 7, 2026 00:24

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github-project-automation bot moved this from Not started to Needs merge in PR Backlog Jan 7, 2026

DanielRosenwasser added 10 commits January 8, 2026 00:41

module -> namespace to avoid merge conflicts

5825123

Accept baselines.

a328f02

Merge remote-tracking branch 'origin/main' into testUpdatesForStrictI…

13ec6c8

…nitialization

Correct some incorrectly-added declare in functions.

f89e695

Add modifications from unsaved files.

a8a7eb6

Accepted baselines.

713c16b

How do I keep missing these?

6b32c4b

Accepted baselines.

b163b67

Fix mistakes.

c68caa0

Fully refresh and accept baselines.

97f1564

Copilot AI mentioned this pull request Jan 8, 2026

[WIP] Update tests for strict initialization #62964

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DanielRosenwasser requested a review from Copilot January 8, 2026 02:13

Copilot AI reviewed Jan 8, 2026

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DanielRosenwasser added 2 commits January 8, 2026 10:49

Remove test line.

bf88bf5

Accept baselines.

3e41c2d

DanielRosenwasser requested a review from Copilot January 9, 2026 05:42

Copilot AI reviewed Jan 9, 2026

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DanielRosenwasser enabled auto-merge January 9, 2026 20:19

DanielRosenwasser requested review from RyanCavanaugh and jakebailey January 9, 2026 20:20

jakebailey approved these changes Jan 9, 2026

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DanielRosenwasser added this pull request to the merge queue Jan 9, 2026

Merged via the queue into main with commit f5ccf43 Jan 9, 2026
33 checks passed

DanielRosenwasser deleted the testUpdatesForStrictInitialization branch January 9, 2026 23:57

github-project-automation bot moved this from Needs merge to Done in PR Backlog Jan 9, 2026

		assignmentCompatWithStringIndexer2.ts(44,9): error TS1184: Modifiers cannot appear here.
		assignmentCompatWithStringIndexer2.ts(45,9): error TS1184: Modifiers cannot appear here.

		assignmentCompatWithNumericIndexer3.ts(31,9): error TS1184: Modifiers cannot appear here.
		assignmentCompatWithNumericIndexer3.ts(32,9): error TS1184: Modifiers cannot appear here.

		assignmentCompatWithNumericIndexer2.ts(30,9): error TS1184: Modifiers cannot appear here.
		assignmentCompatWithNumericIndexer2.ts(31,9): error TS1184: Modifiers cannot appear here.

Test updates for strict initialization #62958

Test updates for strict initialization #62958

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jakebailey commented Jan 9, 2026

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DanielRosenwasser commented Jan 8, 2026 •

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